1. Field of the Invention
The present invention relates to a lateral insulated gate bipolar transistor (lateral IGBT), and particularly, to the structure of a lateral IGBT that influences electron and hole paths in the IGBT.
2. Description of the Prior Art
FIG. 13 is a sectional view showing a lateral MOS gate bipolar transistor, which is hereinafter referred to as the lateral IGBT (insulated gate bipolar transistor) or simply as the IGBT.
The IGBT of FIG. 13 is of an N-channel type and is formed on the primary surface of an N-type epitaxial layer 530. The epitaxial layer 530 contains a low concentration of diffused impurities and is formed on a silicon (Si) monocrystalline substrate. A P-type base region 550 with diffused P-type impurities is formed at the principal surface of the epitaxial layer 530. An N-type emitter region 560 containing a high concentration of diffused N-type impurities is formed at a part of the surface of the base region 550.
A gate oxide film 600 and a gate electrode 540 are formed one upon another to cover a part of the exposed surface of the emitter region 560, a part of the exposed surface of the epitaxial layer 530, and a part of the exposed surface of the base region 550 between these parts of the emitter region 560 and epitaxial layer 530. The base region 550 and emitter region 560 are electrically connected to an emitter electrode E. The gate electrode 540 is connected to a gate electrode G. When a positive voltage exceeding a threshold voltage Vth is applied to the gate electrode G with respect to the emitter electrode E, an N-type inverted layer, i.e., an electron channel is formed under the gate electrode 540. The structure involving the gate electrode 540 and the periphery thereof resembles the structure of a MOSFET (metal oxide semiconductor field effect transistor).
A P-type collector region 510 containing diffused P-type impurities is formed at the primary surface of the epitaxial layer 530 at a predetermined distance away from the base region 550. The collector region 510 is electrically connected to a collector electrode C. The P-type collector region 510, N-type epitaxial layer 530, and P-type base region 550 form a PNP bipolar transistor.
FIG. 14A shows an equivalent circuit of a standard IGBT that is a composite of a PNP bipolar transistor and a MOSFET. A source terminal of the MOSFET is connected to a collector terminal of the PNP transistor, and a drain terminal of the MOSFET is connected to a base terminal of the PNP transistor. Namely, the MOSFET controls a base current of the PNP transistor.
FIG. 15A is a plan view showing the gate electrode 540 and collector region 510 of the IGBT of FIG. 13. A dot-and-dash line A0-A'0 shown in FIG. 15A is the line along which the cross-sectional view of FIG. 13 has been taken.
The collector region 510 has a band shape with round upper and lower ends. The gate electrode 540 has a loop shape to surround the collector region 510 with a predetermined gap between them.
FIG. 15B is an enlarged view showing a part around the dot-and-dash line A0-A'0 of FIG. 15A. The collector region 510 at the right of FIG. 15B is spaced apart from the gate electrode 540 by the predetermined gap. The gate electrode 540 partly overlaps the base region 550 and emitter region 560 that are present below the gate electrode 540. Dotted lines 550a and 560a running in parallel with the gate electrode 540 indicate the boundaries of the base region 550 and emitter region 560.
When locally viewed, the collector region 510, gate electrode 540, base region 550, and emitter region 560 have each a band shape and are arranged in parallel with one another.
A first problem of the conventional IGBT will be explained.
In FIG. 15B, a continuous line 590 represents a path for passing holes serving as first carriers, and a continuous line 580 represents a path for passing electrons serving as second carriers. In practice, these paths are not linear but planar. Namely, electrons flow orthogonally to the long axis of the gate electrode 540 from the emitter region 560 toward the collector region 510, and holes flow vertical to the long axis of the gate electrode 540 from the collector region 510 toward the emitter region 560. These electron and hole paths overlap each other and their directions oppose to each other.
The electron and hole paths and the operation of the conventional IGBT will be explained with reference to FIG. 13. The electron and hole paths are mainly formed at a shallow part of the principal surface of the IGBT.
When a specified voltage is applied to the gate electrode 540, the MOSFET turns on to form an inverted layer, i.e., a channel at the surface of the base region 550 just under the gate electrode 540. Majority carriers, i.e., electrons (e) from the emitter region 560 pass through the channel, enter the epitaxial layer 530, move along the surface of the epitaxial layer 530, and reach the collector region 510, as indicated with a continuous line 580.
On the other hand, holes (h) from the collector region 510 move along the surface of the epitaxial layer 530, enter the base region 550, and reach the emitter electrode E, as indicated with a continuous line 590. At this time, a part of the surface of the base region 550 proximal to the collector region 510 involves the electron channel and emitter region 560, and therefore, the holes must pass under the electron channel and emitter region 560 to reach the emitter electrode E.
Here, the N-type emitter region 560, P-type base region 550, and N-type epitaxial layer 530 form a parasitic NPN bipolar transistor.
The base region 550 where holes pass through has a resistivity that is dependent on the concentration of P-type impurities thereof. When holes pass through the base region 550, they produce resistance R that is proportional to a distance the holes move. The resistance R multiplied by a current that is dependent on the quantity of the holes causes a voltage drop Vt, which is applied to the base and emitter terminals of the parasitic NPN transistor. As the distance for which the holes move in the base region 550 increases, the resistance R increases to increase the voltage drop Vt.
FIG. 14B shows an equivalent circuit of the IGBT including the parasitic NPN bipolar transistor, which is encircled with a dotted line. The parasitic NPN transistor has an emitter terminal connected to the source terminal of the MOSFET, a base terminal connected to the collector terminal of the PNP transistor, and a collector terminal connected to the drain terminal of the MOSFET as well as to the base terminal of the PNP transistor.
If the voltage drop Vt due to the resistance R exceeds a specific value, the parasitic NPN transistor turns on to pass a base current of the PNP transistor. As a result, a current continuously flows through the IGBT irrespective of the gate potential of the MOSFET, to cause a conduction called "latch-up". The latch-up disables the MOSFET to control a current passing through the PNP transistor, to thereby break down the IGBT.
Although the IGBT in the above explanation is of an n channel, the same explanation is applicable to a P-channel IGBT whose regional polarities are opposite to those of the N-channel IGBT. The P-channel IGBT also has the problem of the latch-up.
A second problem of the conventional IGBT will be explained.
The IGBT may be used as a power switching element of a motor. In this case, to reduce power consumption, a saturation voltage Vce between the collector and emitter of the IGBT for providing a specified collector current Ic must be as small as possible.
The saturation voltage Vce is dependent on a resistance value in the epitaxial layer 530 serving as a carrier drift region, and this resistance value is dependent on the concentration of total carriers.
In FIG. 13, the P-type collector region 510 is at the surface of the N-type epitaxial layer 530. This arrangement forms a hole accumulation layer CO around the collector region 510 due to a PN junction at there.
The accumulation layer CO increases the hole concentration of the drift region. To electrically compensate this, the electron concentration of the drift region increases, to thereby increase the concentration of total carriers in the drift region. As a result, the apparent total carrier concentration becomes higher than the impurity concentration of the epitaxial layer 530, and therefore, the resistance of the drift region becomes lower than the resistance of the epitaxial layer 530.
Namely, the carrier accumulation layer CO in the drift region helps reduce the saturation voltage Vce of the IGBT and provides the IGBT with an advantage of low power consumption, compared with a MOSFET, etc., having no carrier accumulation layer. It is necessary, however, to further reduce the power consumption of IGBTs.